PDA is a biomimetic polymer based on mussel adhesive protein, which is excreted by many marine organisms. It is produced by the self-polymerization of dopamine (DA) under oxidative and alkaline conditions, similar to those existing in seawater (Lee et al., 2007). The structure of PDA and the mechanism for its generation closely resemble those of melanin, which is generated from the polymerization of L-dopa.
The adhesive and cohesive properties of PDA are believed to be related to the reactivity of polyorthoquinoneindole, which forms covalent bonds with various substances via Schiff-base type reactions (with amine containing molecules) or Michael type reactions (with amine and thiol-containing molecules) (Scheme 1 in Appendix A). Moreover, the catecholic moiety of PDA can engage in hydrogen bonding, metal complexation, π-π interactions, and quinhydrone charge-transfer complexation (Waite, 1987). Recently, Messersmith et al. reported that PDA could be deposited as a thin adherent polymer film on different material surfaces, including metals, polymers and inorganic materials, converting them to versatile substrates for further ad-layer deposition of various compounds (Lee et al., 2007; Lee et al., 2006; Lee et al., 2009). The interfacial adhesion property of PDA coatings has been widely exploited to introduce new functionalities to materials for various applications (Ye et al., 2011; Chye Khoon et al., 2010; Kang et al., 2012; Chenglin et al., 2012; Sureshkumar et al., 2010; Ren et al., 2011). PDA has recently been utilized to coat different surfaces with antibacterial agents to generate antibacterial surfaces (Shalev et al., 2012).
The unique chemical properties of PDA have inspired researchers to explore its capability to form micro- and nano-capsules for different applications (Cui et al., 2010; Zhang et al., 2011; Ochs et al., 2011; Zhang et al., 2012; Postma et al., 2009; Yu et al., 2009; Cui et al., 2012). Postma et al. have used the template-assisted assembly methodology (Caruso et al., 1998) to polymerize DA on the surface of SiO2 particles to generate hollow PDA nanocapsules after etching the template by acid treatment (Postma et al., 2009). Using a similar templating methodology, PDA micro- and nanocapsules have been constructed to selectively uptake and release charged molecules in response to external pH changes, so paving the way to new and highly specific drug delivery applications (Yu et al., 2009). Monodispersed PDA capsules with a diameter range of 0.4-2.4 μm were also prepared by emulsion templating using oil/water emulsion droplets containing 2% ammonia (Cui et al., 2010; Xu et al., 2011). This method avoids the use of harsh conditions to remove the template, which would otherwise be a limitation when biomolecules are present. The capsules were successfully loaded with functional substances, including magnetic nanoparticles (Fe3O4), quantum dots, and non-aqueous soluble drugs for potential biomedical applications. Using this methodology, PDA capsules whose surface was covalently immobilized with pH-cleavable polymer-drug conjugates were prepared for the intracellular delivery of doxorubicin as an anticancer agent (Cui et al., 2012).
Among various methods being developed for the preparation of micro- and nanocapsules, the sonochemical approach has gained considerable attention. Suslick and coworkers found a remarkably easy sonochemical technique for the preparation of both air-filled micro-bubbles and non-aqueous liquid-filled protein microspheres that were assembled from bovine serum albumin, hemoglobin, and human serum albumin (Suslick and Grinstaff, 1990; Grinstaff and Suslick, 1991; Wong and Suslick, 1995). Free radicals, such as superoxide radicals, generated during sonochemical irradiation (Del Duca et al., 1958; Lippitt et al., 1972), were suggested to be responsible for cross-linking the intermolecular Cys residues through disulfide bond formation, thereby generating the microsphere shell (Suslick and Grinstaff, 1990; Grinstaff and Suslick, 1991; Wong and Suslick, 1995). Similar procedures were also used to prepare microspheres from Cys-less proteins (Avivi and Gedanken, 2002; Dibbern et al., 2006) or even from the polysaccharide chitosan (Skirtenko et al., 2010). Non-covalent intermolecular interactions were suggested to assist microsphere formation.